This is JavaScript, so there are probably tons of weird edge cases, but basically, within a with (expr) { … } block, everything defined on expr is put in scope:

  with (Math) with (console) log(PI)

It reminds me a bit of Clojure's doto.

I have seen it in the wild. I first learned of it from Douglas Crockford’s JavaScript the good parts. He also had some things to say about the new keyword and prototype inheritance and how we should stop using them. Ironically while he was dismissed on that suggestion then, VueJS has pretty much implemented exactly what he has in mind in their V3.

It should be called something like appendixgun then because you don't use it but it still has a chance of causing needless suffering and pain.

There are lots of more conventional footguns in JS, though - everything about the Date object/class, for example.